Cold rolling of steel



United States Patent M corn ROLLING OF STEEL Richard I. Somers,Northville, Mich, and Paul L. Petty, Fort Thomas, Ky; said Somersassignor to Parker Rust Proof Company, Detroit, Mich., a corporation ofMichigan, and said Petry assignor, by mesne assignments, to Acme-NewportSteel Company, Newport, Ky., a corporation of Illinois a No Drawing.Application January 25, 1956 Serial No. 561,355

Claims. (Cl. 148-6.15)

The present invention relates to a method for improving the continuouscold rolling of ferrous strip stock and more particularly relates to aprocess which permits the elimination of the conventional oiling stepprior to rolling and produces a smoother, flatter rolled strip than hasbeen heretofore obtainable. The process includes the provision of aphosphate coating on steel strip stock moving through a conventionalstrip steel pickle line under conventional conditions and then coldrolling that treated strip.

It was early suggested in U. S. Patent No. 2,105,015 that thedeformation of metal was facilitated by providing a phosphate coating onthe metal surface prior to the deforming operation. It has also beensuggested that the drawing of wire, bar stock, tubes and the like ismade easier by phosphate coating the part to be drawn. To the best ofour knowledge there has been no proposal of a process suitable for usein continuous cold rolling of steel strip stock.

Present day cold rolling processing of steel strip stock comprisespreparing the roll surface for cold rolling and then cold rolling in areversing mill or a multiple stand mill. Low carbon steel, silicon steelor other steel strip, which emerges from a conventional hot rolling millusually has some residual rolling scale and corrosion products on itssurface. The preparation for cold rolling normally includes processingthe hot rolled strip to clean the surface by passing it through a pickleline of conventional form which may, for example, include in sequenceone or more mineral acid dip stations, one or more water rinse stationsand an oiling station. In certain instances Where the mill scale isheavy, the pickle line may include mechanical abrading stations oralkali stations prior to the acid dip stations. The steel strip iscontinuously passed through these sequential stages and is recoiledafter oiling to await the cold rolling operation. The rolling mill,whether reversing or multiple stand, operates at speeds and underconditions which make it, impossible to feed directly from the pickleline to the rolling line. Because of this necessary but variable lengthdelay between thepickling of the strip and its rolling, the corroding ofthe pickled strip becomes an important problem. It is for this reasonthat most pickle line's include a final oiling station The oiling stepis highly undesirable. from the dual standpoints that it drips from thepickled coils, makes the working area slick, dirtyand unsafe for workmenand that-after cold'rolling when the cold rolled strip is annealed theoil residue produces a surface coating of undesirable smut. i

It will be apparent that in order to produce sufiicient pickled steelstrip to keep a cold rolling mill in continuous production, the pickleline must necessarily operate continuously and at relatively rapidspeeds in order to be economic. Although actual speedsnec'essarily'varyv in dilferent installations, pickle line speeds in therange of l 150 feetper minute to 450 feet per minute are common.

One of the difficulties in supplying a process for continu- 2,859,145Patented Nov. 4, 1958 2 ously forming a phosphate coating on the'stripsteel moving through a pickle line is to provide a solution which willform the necessary coating in the limited time which is available forcontact of the strip surface with the phosphate coating solution. Forexample, if the pickle line moves at feet/minute a phosphate coatingtank having a 25 foot length provides only about 10 seconds contact timefor the strip. It will be apparent that as the pickle line speedincreases, either the phosphate coating tank must be greatly lengthenedor the coating solution must be adapted to act in a much shorter timethan 10 seconds, for example in 3 to 8 seconds.

in the rolling of strip through a reversing mill or in a multiple standmill, the most important objective is to produce rolled strip of uniformthickness or gauge having good surface finish. The speed of travel of asteel strip through a cold rolling mill will vary, reversing mills beinggenerally slower than multiple stand mills. In order to be economic, thestrip speed should be above about 600 feet/minute and preferably aboveabout 1000 feet/minute with speeds as high as 50007500 feet per minutebeing obtainable on present day multiple stand rolling mill equipment.It will be appreciated that variations in gauge of the resulting stripcan only be accomplished by varying the spacing between the rolls and ateconomic cold rolling speeds only extremely minor adjustments of theroll spacing can be made while the strip is passing through the rolls.In accordance with this invention it has been found that when variationsin the weight or thickness of the phosphate coating on the surface ofthe strip exist, the rolling mill operator experiences difiiculty inmaintaining gauge. When the phosphase coating weight varies widely frompoint to point along the strip, attempts by the operator to correct forthese variations may cause side slipping or ultimate ripping and tearingof the strip. When a thick portion is observed by the operator the rollpressure is increased in an attempt to hold gauge and as the thickerportion of the strip passes through the rolls the extra roll pressurecauses the rolls to grab the following portion of the strip and thuscause ripping or tearing. This problem represents the most formidabledifliculty which must be overcome in providing a continuous phosphatecoating process for steel strip. It was further found that when usingconventional phosphate coating solutions and normal delays in processingin the pickle line occurred, that portion of the steel strip in contactwith the phosphate coating solution during the delay received a heavier,and thusthicker phosphate coating than other portions ofthe strip whichpassed through the solution at the normal pickle line speed. Normaloperation of a pickle line necessarily involves variation in strip speedto permit the tacking together of the ends of coils and of course thereare unexpected mechanical delays which accompany any continuousoperation. It was thus found tobe necessary to provide phosphate coatingsolutions which would function to produce the necessary quantity ofphosphate coating in a short time, e. g., less than 15 seconds, butwhich would nevertheless avoid the deposition of widely variant weightcoatings even though a pickle line stoppage, measured in minutes,occurred.

The primary objective of this invention is to provide a process whichenables the continuous coating of steel strip with a phosphate coatingof suflicient uniformity in thickness to facilitate cold rolling, theuniformity enabling the holding of gauge during continuous cold rolling.A further object of this invention is to provide a process for treatingferrous strip stock continuously in a pickle line which produces aphosphate coating which aids in the subsequent cold rolling and whichleaves on the sur: face a suflicient quantity of phosphate residue toprevent sticking when the cold rolled strip is annealed either instacked sheet or coil form.

Another important objective ofthis invention is to provide phosphatecoating solutions capable of producing on a continuously moving ferrousstrip a coating of the necessary Weight or thickness and of suflicientuniformity to permit the attainment of the desired gauge tolerance inconventional cold rolling.

Another object of this invention is to provide a process forcontinuously forming a corrosion-resistant phosphate coating on thesurface of ferrous strip subsequent to an acid pickle which enables theelimination of the application of an oil coating after acid pickling andprior to cold rolling, which heretofore has been conventional in thisart.

Still another object of this invention is to provide a process forcontinuously applying a phosphate coating to a ferrous strip movingthrough a pickle line which is of sufficient uniformity to facilitatesubsequent continuous cold rolling while maintaining the desired gaugetolerance and which reduces the power required to effect a predeterminedamount of reduction; to provide such a phosphate coating as to enablethe cold rolling of ferrous sheet of unusual flatness.

Another object is to provide such a phosphate coating to enable theelimination of the use of a roughened surface roll such as a Pangbornroll in a reversing mill or in the last stand of a multiple stand milland yet produce a cold rolled sheet of suitable surface condition topresent sticking on annealing.

Yet another object is to provide such a phosphate coating as to enablethe annealing of sheets of such cold rolled strip in stacks up to 100%higher than heretofore used practice and at temperatures up to about1750 F.; to provide such a phosphate coating as to enable the subsequentstamping, in the hardened cold rolled condition of various parts such aselectric motor laminations, which after assembly may be annealed withoutsticking at temperatures as high as about 1750 F.

In accordance with the present invention it has been found that theabove objectives may be realized if certain phosphate coating solutionsare used under relatively closely controlled conditions in operation. Ithas been found that certain other phosphate'coating solutions are eitherof minor benefit or are unsatisfactory for the purposes of thisinvention. Broadly stated the method of this invention comprises thesteps of removing scale and corrosion products from the surface offerrous strip stock, contacting the cleaned and continuously movingsurface of the strip with an aqueous acidic zinc phosphate coatingsolution. comprising as the essential coating producing ingredients inweight percent, 0.18 to 0.9 zinc, 0.5 to 2.0 P0 0.7 to about 2.5 C10 andhaving a total acid of 10-.60 points, and thereafter cold rolling thecoated strip- Somewhat more desirable results are obtained if thesolutions include as optional ingredients 0.1 to 2.4 N0 and 0.17 to0.75% calcium by weight. By total acid of 10- 60 points is meant that a10 ml. sample of the solution requires 10-60 ml. of one-tenth normalsodium hydroxide to reach a phenolphthalein endpoint. It is satisfactoryto employ a solution as above defined but containing the proportions ofzinc and N0 which would result from adding zinc nitrate to the solutionto reach the maximum total acid of 60 points so long as the ratio ofcalcium to zinc is maintained in the range of 0.421 to 1.111.

When the above proportions of constituents and the coating weight doesnot exceed 200 mg./sq. ft. from the materially improve the rollingcharacteristics and after rolling do not leave a sufiicient quantity ofphosphate residue to prevent sticking on annealing, whereas coatingWeights above about 1000 mg./sq. ft., even though substantially uniform,make it difficult to maintain the desired gauge upon rolling. When thecoating is substantially uniform and in the preferred weight range, thecoated strip may be rolled in a series of passes to reduce the thicknessto one-sixth the original thickness and yet sufficient phosphate residueremains after rolling to, prevent sticking when sheets stacked fiftyinches high areannealed at temperatures as high as about 1400 F. It hasbeen observed that the presence of the residual phosphate on the surfaceafter continuous cold rolling permits annealing at 100 F.200 F. higherthan temperatures heretofore utilizable so long as the pressure on theadjacent surfaces is comparable.

Due to the above referred to uncontrollable and unexpected stoppageswhich occur in the pickle line during normal operation, the relativeproportions of ingredients in the above solutions have been found to becritical. When the proportions fall outside the limits given, thevariation in coating weight which is obtained is so great that onsubsequent rolling, difficulty is experienced in maintaining the desiredgauge tolerance, or side slipping, ripping or tearing of the strip isencountered.

The best over-all results have been obtained from the use of solutionscontaining calcium. When calcium is present the ratio of calcium to zincin the solution: should be maintained within the range of 0.4 to 1 up to1.1 to 1. Moreover, in these calcium-containing solutions, while it ispossible to use C10 concentrations .as low as 0.15% it is desirable tomaintain the chlorate concentration above'about .3% and preferablybetween 0.4-0.6. The calcium containing solutions appear to produce afiner grain coating and this coating provides the maximum aid duringrolling while concurrently producing the best surface finish.

After the coated strip emerges from the phosphate coating solution, itmay, optionally, be rinsed in.a conventional dilute aqueous solution ofchromic acid or the equivalent. It is then recoiled on a receivingmandrel in a conventional manner. Rolling may then be effected on areversing or multiple stand mill by employing the conventionally usedlubricants during the rolling operation. Any of a number of satisfactorylubricants may be employed including oils, such as mineral oil, palmLubricants for this purpose are 1 preferably used in aqueous form,either as a solution 1 or emulsion. Dilute aqueous solutions consistingof, for

oil, rape seed oil, etc.

example, 1% to 3% oil and the balance water, of a water soluble mineraloil, or a lard base oil together with an emulsifying agent, have beensatisfactorily em ployed. One specific aqueous base lubricant which hasbeen used with unusual success is a mixture of lard oil and a fatty acidand has the following analysis: Fatty acid (calculated as oleic acid),12.9% by weight, iodine number 36.9 and a saponification number of 32.4.A small amount of an emulsifying agent to make this mixed oil watersoluble is added to a solution containing'1% 2% of this mixed oil,balance water.

Hot rolled strip may be satisfactorily pickled in a pickling lineconsisting only of an aqueous sulfuric acid solution followed by a waterrinse. Conventional acid pickling solutions, rinsing procedures and thelike may be employed but it is desirable to process the hot rolled stripthrough an acidic pickling solution prior to its introduction into theabove disclosed phosphate coating solutions. It'is important to avoidthe contamination of the phosphate coating solution by the picklingacids and is therefore desirable to thoroughly rinse the strip emergingfrom the sulfuric acid pickle before its introduction into the phosphatecoating solution. Where heavy mill scale is present on the hot rolledstrip or other strip to .be phosphate coated, conventional mechanicalabrading means or molten alkali baths may be employed, prior to the acidpickling.

One unexpected advantage which accrues from the practice of the methodof this invention is that it is unnecessary to employ shot blasted orroughened rolls in a reversing type mill to attain a surface finish thatwill not stick during coil or stack annealing It is also unnecessary toemploy a similar roll as the last position roll in a multiple standmill. 7 p

The invention is illustrated in greater detail in the examples givenbelow. It is to be understood that the examples given are intended toillustrate operative conditions only and are not intended to limit theinvention to the specific proportions of ingredients set forth since thecritical limits thereof have been hereinbefore.

Example I An aqueous acidic phosphate solution tank was installed as thelast station in a sulfuric acid pickling line comprising a sulfuric acidpickling station, a water rinse and the phosphate coating solutionstation. An aqueous acidic zinc phosphate solution was prepared in thephosphate station and analyzed -to contain in percent w./v.;chlorate0.17, nitratel.7, zinc-0.5, PO 1.3, calcium-0.5; the bathoperated at a total acid of 25-30 points and a free acid of 4-5. Thebath was maintained at a temperature of about 150 F.-180 F. and hotrolled low carbon steel strip was fed throughthe sulfuric acid pickle,water rinsed and through the above solution at the rate of about to 120feet per minute which provided a contact time in the coating solution ofabout 10-12 seconds. This processing was continued for about 8 weeksduring which time about 22,500,000 square feet of steel strip wasprocessed through the solution. The coating weights obtained during thisperiod had average weights between about to mg./ sq. ft. with Variationstherefrom ranging from about 50 mg./sq. ft. to about 200 mg./sq. ft. Thestrip emerging from the phosphate coating solution was coiled inconventional manner. In some cases the coils were immediately rolled andin others a time delay of one or more days occurred before the rollingwas done. stock was rolled under conditions which reduced the initialthickness from 0.1" to 0.068" in one pass, to 0.05" in two passes, to0.038" in three passes, to 0.029" in four passes and 0.025" in fivepasses. rolled on a Bliss rolling machine of the reversing type andduring the rolling there was a continuous flooding of the rolls and thework with an aqueous base lubricant. During this period of operationthere were shutdowns in the pickle line travel for periods exceeding tenminutes and yet the variation in coating weight was insuflicient toprevent the coil from being continuously cold rolled. Some of the coatedstock was stamped directly into small horsepower motor laminations inthe cold rolled hardened condition and it was observed that there wasless wear on the dies when the stamping was done in the cold rolledhardened condition than otherwise. Other portions of this rolled stripwere annealed in air and deoxidizing atmosphere furnaces in coils and instacked sheet form at temperatures ranging from 1200 F. to as high as1650 F. without encountering sticking between successive layers of thecoil or between the sheets. Some of these stacks were increased to ashigh as 60 inches, from the normal 36" height and annealed at 1275 F.without sticking.

During the operation of the above solution, sodium Most of this treatedstrip The strip was a chlorate was added to raise the chlorate contentto 0.32. The coating resulting from this solution was uniform inappearance and appeared to increase in weight less on line stoppage thansolutions containing the lower chlorate percentages. Additional chloratewas added to raise the concentration to 0.38% and an even greateruniformity Another solution, which does not contain calcium, which wasfound to be satisfactory was prepared in the phosphate coating stationand analyzed to contain in percent w./v.: chlorate-'-0.43%, zinc0.7%,P0,; 1.1%, total acid 26.2%. Low carbonhot rolled strip operating in thesame line as that described in Example I was processed through the 'bathof the above composition at a temperature of about F. F. untilapproximately 370,000 square feet of surface had passed through thebath. The coatings obtained in a contact time in the bath of 10-12seconds were observed to be of conventional gray appearance anduniformly distributed over the surface. Typical coating weights obtainedwere about 188-205 mg./sq. ft. With chlorate concentration above 0.7% asubstantially uniform coating was obtained even though the line stoppedfor times measured in minutes.

The coated strip was rolled while an aqueous base lubricant was flowingover the surface of the rolls and the strip. No trouble was encounteredduring the rolling, and the rolled strip upon examination revealed auniformly flat surface having a smooth surface finish and a cleanmetallic appearance.

Example III has been processed through the bath. Coating weights whichresulted from this treatment varied from 264- 370 mg./sq. ft. During theoperation of this bath the chlorate content of the solution wasincreased in steps to .6%, .7% and gradually to 2%. As the proportion ofchlorate exceeded 0.7% it was noted that line shutdowns had less' effecton the increase in the resulting coating weight. The resulting coilswere rolled in accordance with the procedure stated in Example I andsatisfactory rolling conditions were obtained with the resulting rolledstrip having uniformly smooth surface, being flat and having theappearance of clean metal.

Example IV An aqueous acidic zinc solution was prepared, placed in thephosphate coating station and analyzed to contain in percent w./v.:zinc-0.55, PO 0.82, N0 1.17. Sodium nitrite was added to the solution toproduce a concentration of 0.0005% and maintained ator above thatconcentration during use. The bath had a total acid of 20 points. Thebath was maintained at a temperature of F.- F. and hot rolled low carbonsteel strip was processed through the bath in the line and under similarconditions described in Example I. When the coil was moving through thebath at the regular speed of 100-420 feet/minute, the coating weightsobtained varied from about 150-200 mg./'sq. ft. When the pickle line Wasstopped'for, a period of ten minutes the coating weight on'the stripwhich was emerged in the tank during this sto'ppage'was in the range of800- 1000 ing/sq. ft. Upon rolling this coil it was found that thisheavy phosphate coating weight area caused slipping and tearing of thestrip as it passed through the rolls. i

What is claimed is:

1. A method for treating ferrous the steps of pickling the strip,contacting the cleaned strip while the same is moving with an aqueousacidic zinc phosphate solution comprising as the essential coatingproducing ingredients in weight percent, L0.18-0.9 zinc, 0.5-2.0 P0 and0.7-2.5 ClO said solution having a total acid in the range of 10-60points, maintaining said strip in contact with said solution until aphosphate coating is formed on the surface. of said strip, said coatinghaving a Weight in the range of about 50 mg./per sq. ft. to about 1000mg./per sq. ft. and varying from the average coating Weight prevalent onsaid strip by not more than about 200 rug/per sq. ft., and thereaftercontinuously cold rolling the said strip.

2. A method for treating ferrous strip which comprises the steps ofpickling the strip, contacting the cleaned strip while the same ismoving with an aqueous acidic zinc phosphate solution comprising as theessential coating producing ingredients in weight percent, 0.18-0.9zinc, 0.5-2.0 P0,, 0.1s 2.5 010,, 0.1-2.4 N0 0.17 0.75 calcium, saidsolution having a total acid in the range of 10-60 points, controllingthe temperature so as to produce a coating having a weight in the rangeof 50-1000 mg./per sq. ft. and varying from the average weight prevalenton said strip by not more than about 200 mg./per sq. ft., and thereaftercontinuously cold rolling the said strip.

3. A method for treating ferrous strip which comprises the steps ofpickling the strip, contacting the cleaned strip while the same ismoving with an aqueous acidic zinc phosphate solution comprising as theessential coating producing ingredients in weight percent, 0.18- 0.9zinc, 0.5-2.0 P0 0.15-2.5 C10 0.1-2.4 N0 0.17- 0.75 calcium, saidsolution having a total acid in the range of 10-60 points, controllingthe temperature so as to produce a coating having a weight in the rangeof 50-1000 mg./per sq. ft. and varying from the average coating weightprevalent on said strip by not more than about 200 mg./per sq. ft.,rinsing said coating strip in a dilute aqueous chromic acid solution andthereafter cold rolling said strip.

4. A method in accordance with claim 2 wherein said chlorate is presentin a concentration between 0.3 and 0.6%.

5. A method in accordance with claim 1 wherein the strip which comprisescontinuousrolling is performed while continuously flooding said stripwith an, aqueous base lubricant.

6. A method in accordance with 'claim2 wherein said rolling is performedlv'vhile; said strip is continuously flooded with a dilute aqueouslubricant consisting essentially of 97%"99%. water and 1-3% of a mixtureof fatty acid and an emulsifying agent, said fatty acid containing theequivalent of 12.9% by weight of 'oleic acid,

having an iodine number of 36.9 and a .saponifica'tion' number of 32.4.r

7. A method in accordance with claim 2 wherein said coating has a weightin the range of about 150-400 mg./per sq. ft. 1

8. A method in accordance with claim 2 wherein the ratio of calcium tozinc in the solution is within the range of 0.4-1 and 1.1-1. i

9. A ferrous strip which may" readily be continuously cold rolled andwhich has an acidic phosphate coating thereon, said coating having aweight in the range of a pickling thereof, with an aqueous acidic zincphosphate". solution comprising as the essential coating producing;

ingredients in weight percent, 0.18-0.9 zinc, 0.5-2.0 P0

and 0.7-2.5 C10 0.1-2.4 N0 0.17-0.75 calcium, said solution having atotal acid in the range of 10-60 points! said phosphate coating having aweight in the range of about 50 mg./per sq. ft. to about 1000 mg./persq. ft. and varying the average coating weightprevalent'onjsaid; stripby not more than about 200 mg./per sq. ft.

References Cited in the file of this patent UNITED STATES PATENTS '19402,191,435 Ballardet a1. Feb. 27, 2,230,319 -Canzler Feb. 4, 19412,293,716 Darsey Aug. 25,1942 2,540,314 Amundsen Feb. 6 1951 2,724,668Russell Nov. 22, .1955 2,743,204 Russell Apr. 24,

OTHER REFERENCES Streicher: Phosphatization of Metallic Surfaces, MetalFinishing, pages 61-69, August 1948 (page 66 re-:. lied on).

1. A METHOD FOR TREATING FERROUS STRIP WHICH COMPRISES THE STEPS OFPICKLING THE STRIP, CONTACTING THE CLEANED STRIP WHILE THE SAME ISMOVING WITH AN AQUEOUS ACIDIC ZINC PHOSPHATE SOLUTION COMPRISING AS THEESSENTIAL COATING PRODUCING INGREDIENTS IN WEIGHT PERCENT, 0.18-0.9ZINC, 0.5-2.0 PO4 AND 0.7-2.5 GLO3, SAID SOLUTION HAVING A TOTAL ACID INTHE RANGE OF 10-60 POINTS, MAINTAINING SAID STRIP IN CONTACT WITH SAIDSOLUTION UNTIL A PHOSPHATE COATING IS FORMED ON THE SURFACE OF SAIDSTRIP, SAID COATING HAVING A WEIGHT IN THE RANGE OF ABOUT 50 MG./PER SQ.FT. TO ABOUT 1000 MG./PER SQ. FT. AND VARYING FROM THE AVERAGE COATINGWEIGHT PREVALENT ON SAID STRIP BY NOT MORE THAN ABOUT 200 MG./PER SQ.FT., AND THEREAFTER CONTINUOUSLY COLD ROLLING THE SAID STRIP.